If you are unable to find the part at a Mazda dealer, you can try to locate an aftermarket or reconditioned bumper cover from an auto collision parts store. Since I don't know where you are, you can try Keystone.
http://www.keystone-auto.com/
Keystone is the largest aftermarket automotive parts supplier in the US. Because the bumper cover is not a safety item (it provides appearance and aerodynamics, it's made out of plastic) you should not have any reservations about using an aftermarket or reconditioned part.
You will probably find that you will also save considerable money by purchasing an aftermarket or reconditioned part.
I suggest that you make an appointment with your local collision repair shop to search for parts and complete your repair. A bumper cover replacement should run in the neighborhood of $600-$800 parts/labor included.